Anyone who has spent any time with me is familiar with my complete and utter loathing of everything Intuit. But instead of ranting about how it's an inferior, bloated, poorly-coded application that ignores both IT and accounting best practices, I'm going to provide some hopefully useful information on how to get QB 2007 to work in a network environment when you're moving up from QB 2006.
Like most of us in the SMB space, we have a lot of customers running QuickBooks. We were eventually able to get virtually all of our SBS customers who purchased QB2006 running with the data living on their server. Of course, this required installing the complete QB2006 app on the server - which I wasn't exactly thrilled about. As you may know, with QuickBooks 2007 Intuit has actually started making some progress towards a network friendly app. Now, I don't know if someone at Intuit started thinking - or if maybe Susan's 2x4 finally hit home. Regardless - with QuickBooks 2007 it now has a server install option (we no longer need to install the full app on the server), and it also supports running as a non-admin. Who'da thunk? 
So, I've now moved three different clients from QuickBooks Pro 2006 to QuickBooks Pro 2007. Since I didn't like having the full QB app on the server, and I now no longer have to - I decided to completely uninstall QB2006 from the server, then run the server install of QB2007. However, when I installed QB2007 on the clients, they couldn't open the company files on the server. After digging around for a little bit - I found the culprit: Intuit. Believe it or not, the QuickBooks 2006 uninstall routine did not remove the QuickBooksDB service. Of course, QB2007 installs its own new data service (QuickBooksDB17) - so I was left with two QB data services running and not playing nice with each other. Since I've seen this with every upgrade I've done so far, I figured there is a chance someone else might run in to it
The simple solution is to stop both services, remove the old QuickBooksDB service, then start the new QuickBooksDB17 service and you're good to go.
If you need a refresher on removing Windows services - download the Windows 2003 Resource Kit Tools here. Once you've installed the reskit tools on your server, click on Start | Programs | Windows Resource Kit Tools | Command Shell then enter the following command:
INSTSRV QuickBooksDB REMOVE
Start the new QuickBooksDB17 service and voila!